Synthesis of raltegravir

ABSTRACT

The present invention relates to a novel synthetic route for the preparation of raltegravir and pharmaceutically acceptable salts, starting from 2-amino-2-methylpropanenitrile and oxadiazole carbonyl chloride, through the formation of a pyrimidinone intermediate of formula (V).

FIELD OF THE INVENTION

The present invention relates to a novel and cost-effective method forthe preparation of Raltegravir of formula (I) conforming to regulatoryspecifications. Specifically, the method circumvents the formation ofassociated impurities, which are encountered in prior art methods.

BACKGROUND OF THE INVENTION

Raltegravir of formula (I ), chemically known asN-[(4-fluoropheny)methyl]-1,6-dihydro-5-hydroxy-1-methyl-2-[1methyl-1-[{(5-methyl-1,3,4-oxadiazol-2-yl)carbonylamino]ethyl]-6-oxo-4-pyrimidinecarboxamide is a humanimmunodeficiency virus (HIV) integrase strand transfer inhibitor whichin combination with other antiretroviral agents is indicated for thetreatment of HIV-1 infection in adult patients already undergoingtreatment and are having evidence of viral replication and HIV-1 strainsresistant to multiple antiretroviral agents. The potassium salt ofRaltegravir having proprietary name Isentress is administered orally asa tablet of 400 mg strength.

Raltegravir (I) Raltegravir Potassium (Ia)

Various researchers have attempted to synthesize the activepharmaceutical ingredient of formula (I) and its pharmaceuticallyacceptable salt. Journal of Medicinal Chemistry 2008, 51, 5843-5855 aswell as WO 2006060730 discloses a method for the preparation ofraltegravir starting from 2-amino-2-methylpropanenitrile. However, themethod has a serious drawback, involving formation of up to 22% ofO-methyl impurity during the N-methylation of the pyrimidinone ring withMg(OCH₃)₂. The removal of O-methylated impurity requires severalpurifications involving recrystallization and/or column chromatography,which ultimately reduces the overall yield considerably. Further, thesynthetic route also results in the formation of another associatedimpurity during the preparation of raltegravir free base due to theO-acylation of the penultimate intermediate with oxadiazolecarbonylchoride. Thus the formation of several associated impurities andtheir removal either by chromatographic purification or by successivecrystallizations results in loss of yield which ultimately makes theprocess unviable for commercial use.

Organic Process Research & Development 2011, 15, 73-83 provides anothercircuitous method for preparation of raltegravir, wherein two methodshave been disclosed for the preparation of hydroxy pyrimidinone. Thedocument mentions that the yield of hydroxy pyrimidinone is dependent onthe E/Z configuration of the compound obtained by reaction of amidoximewith dimethyl acetylene dicarboxylate. Higher yield (72%) is obtainedwith the Z isomer while lower yield (48%) is obtained with thecorresponding E-isomer. Further, the method involves additional steps ofprotection and deprotection of the amino and hydroxyl group thereby,making the method lengthy and less attractive for industrial scale.

The reference also reports another method for the methylation of hydroxypyrimidinone with trimethyl sulfoxonium iodide/magnesium hydroxidewithout any mention about the reaction solvent and discloses that thereaction provides ≈99% of the desired N-methyl compound after prolongedheating of the reaction mixture at high temperatures, which in turn islikely to give rise to impurities.

Thus the prior art methods are associated with lengthy and circuitoussynthetic routes, which not only consume more time for each batch runbut also generates associated impurities thereby requiring additionalsteps of purification. Regulatory authorities all over the world havevery stringent norms for permissible limits of such impurities in eitherthe active ingredient or the final formulation.

Therefore, it was imperative for the synthetic chemists to control suchimpurities below detectable limits during synthesis and develop a routeinvolving a cost-effective process which did not require additionalsteps of purification. Hence, to overcome the prior art drawbacks, thepresent inventors developed a new synthetic route for raltegravir, whichinvolves lesser number of synthetic steps, controls the level ofundesired impurities below permissible limits and is cost-effective,environmental friendly for convenient implementation on an industrialscale.

OJECTS OF THE INVENTION

The main object of the invention is to provide an industrially viablemethod for preparing raltegravir and its pharmaceutically acceptablesalts, conforming to regulatory specification.

Another object of the present invention is to provide a novel syntheticroute for preparation of raltegravir and its pharmaceutically acceptablesalts, which circumvents the formation of impurities and undesiredisomers encountered in prior art.

SUMMARY OF THE INVENTION

The main aspect of the present invention is to provide a convenientsynthetic route for the preparation of raltegravir and its salts, whichconfirms to regulatory specification.

Another aspect of the present invention is to provide a method forpreparation of raltegravir comprising

-   -   i) reaction of 2-amino-2-methylpropanenitrile with oxadiazole        carbonylchloride to give        N-(2-cyanopropan-2-yl)-5-methyl-1,3,4-oxadiazole-2-carboxamide        (II);    -   ii) treating        N-(2-cyanopropan-2-yl)-5-methyl-1,3,4-oxadiazole-2-carboxamide (II)        with hydroxyl amine (IIIa) or N-methyl hydroxylamine (IIIb) to        give        N-[(1Z)-1-amino-1-(hydroxyimino)-2-methylpropan-2-yl]-5-methyl-1,3,4-oxadiazole-2-carboxamide        (IVa) or        N-{1-[hydroxyl(methyl)amino]-1-imino-2-methylpropan-2-yl}-5-methyl-1,3,4-oxadiazole-2-carboxamide        (IVb),    -   iii) reaction of compound of formula (IVa) or (IVb) with dialkyl        acetylene dicarboxylate to give        methyl-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate        (Va),    -   iv) reaction of compound of formula (Va) with        p-fluorobenzylamine to give        N-(4-fluorobenzyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide        (VI),    -   v) methylating compound of formula (VI) with a methylating agent        to give raltegravir (I).

Yet another aspect of the invention is to provide a method which isefficient, cost effective, environmentally friendly and industriallyviable.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have developed a novel synthetic route whichovercomes the serious drawbacks of prior art such as:

-   -   (i) formation of undesired O-methylated derivatives during        methylation of the pyrimidinone nitrogen;    -   (ii) circumventing the preparation of a mixture of E/Z isomer        obtained by reaction of an amidoxime derivative with dialkyl        acetylene dicarlomylate;    -   (iii) reducing the number of steps for preparation of        raltegravir thereby making the process cost effective and        considerably reducing the time taken for each batch run and    -   (iv) obtaining raltegravir of desired purity without the        formation of undesired impurity formed in the prior art.

One embodiment of the present invention relates to a novel syntheticroute for preparation of raltegravir and its salts. The syntheticsequence for preparation of raltegravir is represented in Scheme I.

The present invention relates to a novel synthetic route for preparationof raltegravir comprising reaction of 2-amino-2-methylpropanenitrilewith oxadiazole carbonyl chloride in presence of a base. The latterreagent acts as a protecting group for the amino functional group anddoes not require to be deprotected at any stage of raltegravirsynthesis.

The protection of the amino group which is carried out in presence of abase utilizes a halogenated hydrocarbon as solvent such asdichloromethane, dichloroethane, chloroform etc. The solvent ispreferably dichloromethane.

The base utilized is selected from an organic base like N-methylmorpholine (NMM), pyridine, N,N-dimethylamino pyridine (DMAP) etc. Theamount of base used is in the range of 0.99 to 1.50 moles per mole of2-amino-2-methylpropanenitrile used in the reaction.

After completion of reaction at a temperature ranging from −5° C. to+10° C., the reaction mixture was concentrated and theN-(2-cyanopropan-2-yl)-5-methyl-1,3,4-oxadiazole-2-carboxamide (II) wasseparated out from the residue by addition of an alcohol such asmethanol, ethanol, isopropanol to the residue.N-(2-cyanopropan-2-yl)-5-methyl-1,3,4-oxadiazole-2-carboxamide (II) isobtained with yield in the range of 90 to 95%.

The amino protectedN-(2-cyanopropan-2-yl)-5-methyl-1,3,4-oxadiazole-2-carboxamide (II) isthen treated with a hydroxylamine of formula (IIIa) or its methylderivative of formula (IIIb) to give the respective amidoximeN-[(1Z)-1-amino-1-(hydroxyimino)-2-methylpropan-2-yl]-5-methyl-1,3,4-oxadiazole-2-carboxamide(IVa) or imine derivativeN-{1-[hydroxyl(methyl)amino]-1-imino-2-methylpropan-2-yl}-5-methyl-1,3,4-oxadiazole-2-carboxamide(IVb).

The reaction was carried out in alcohol as solvent. The alcohol wasselected from the group comprising of methanol, ethanol, isopropanoletc,

Hydroxylamine (IIIa) or N-methyl hydroxylamine (IIb) was added to themixture and the resultant mixture heated at 45 to 60° C. till completionof reaction. The reaction mixture was cooled to 0° C. and diluted with anon-polar solvent selected from the group comprising of an aliphatichydrocarbon, aromatic hydrocarbon, which in turn was selected from thegroup comprising of hexane, cyclohexane, heptane, toluene, xylene etc.

The product of formula (IVa) or (IVb) separating out quantitatively wasfiltered and dried.

N-[(1Z)-1-amino-1-(hydroxyimino)-2-methylpropan-2-yl]-5-methyl-1,3,4-oxadiazole-2-carboxamide(IVa) was then treated with dialkyl acetylene dicarboxylate and cyclizedto provide pyrimidinone methyl2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate(Va).

The compound of formula (IVa) was added to an alcohol selected from thegroup comprising of methanol, ethanol, isopropanol etc and refluxed tillcompletion of reaction.

The mixture was cooled between 0 and 30° C. and dimethylacetylenedicarboxylate was added gradually to the mixture.

The amount in moles of dimethylacetylene dicarboxylate added withrespect to compound of formula (IVa) was between 0.9 moles and 2.0moles.

The reaction mixture was stirred for 2-3 hours and concentrated. Theresidue was diluted with an aromatic hydrocarbon selected from the groupcomprising of toluene, xylene etc and heated between a temperature of110° C. and 140° C. till completion of reaction.

The mixture was then cooled between 50 and 70° C. and diluted with amixture of an alcohol and ether.

The alcohol was selected from the group comprising of methanol, ethanol,isopropyl alcohol etc while the ether was selected from the groupcomprising of dimethoxyethane, tetrahydrofuran, diisopropyl ether,methyl tert-butyl ether etc.

Compound of formula (Va) thus separating out was filtered and dried.

Methyl-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate(Va) on amidation with p-fluorobenzyl amine in presence of a base gaveN-(4-fluorobenzyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide(VI).

The compound of formula (Va) was converted to raltegravir by suspendingit in an alcohol selected from the group comprising of methanol,ethanol, isopropyl alcohol etc.

4-Fluorobenzyl amine was added to the mixture and stirred between 75-90°C.

The amount of 4-Fluorobenzyl amine added was between 1.0 mole and 2.5moles per mole of compound of formula (Va).

After completion of the reaction, the mixture was cooled between 55 and75° C. and quenched with aqueous acetic acid. The demethylatedRaltegravir intermediate of formula (VI) separated out, which wasfiltered and washed.

N-(4-fluorobenzyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide(VI) was then alkylated with a methylating agent to give raltegravir offormula (I).

The methylating agent was selected from the group comprising of methyliodide, trimethylsulfoxonium iodide, formic acid/formaldehyde, methyltrifluoromethanesulfonate, dimethyl sulfate, dimethyl zinc,trimethyloxonium tetrafluroborate.

In a specific embodiment, the raltegravir intermediate of formula (VI)was treated with a methylating agent such as trimethysulfoxonium iodideat 100° C. till completion of reaction and cooled between 5 and 25′C.The reaction mixture was quenched with aqueous sodium bisulfate andextracted with a chlorinated solvent such as dichloromethane. Theorganic layer was separated, concentrated and the residue wascrystallized from aqueous methanol to provide raltegravir (I) having thedesired purity.

Among the advantages of the novel route of synthesis of raltegravir isthe regioselective methylation ofN-(4-fluorobenzyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide(VI) obtained by amidation ofmethyl-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate(Va) with p-fluorobenzyl amine. The methylation reaction providesregioselectivity of more than 99% as against methylation of oxygen at6^(th) position, thereby resulting in a higher yield of more than 90%.The yield for the methylation reaction is in the range of 88 to 90%.

On the other hand, prior art methods provide a 7:3 ratio of the1-nitrogen methylated: 6-oxygen methylated product after methylation.

Further, the protection of 5-hydroxy group with pivaloyl chloride orbenzoic anhydride before methylation reaction is avoided. The prior artmethod requires additional step for deprotection which increase the costand time for each batch run.

Thus, the invention is free from the additional steps of protection anddeprotection of amino protecting group and also provides a simple,economically and efficient method for preparing raltegravir with desiredpurity.

The preparation of pyrimidinone methyl2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate(Va), by treating with diallyl acetylene dicarboxylate and cyclizing hasan advantage over prior art, as the yields were increased by 10 to 15%.

Thus, the present invention based on the above advantages satisfies theneed for, developing an alternate synthetic route for raltegravir whichhas minimum number of steps and cost effective and environmentalfriendly and easily implemented industrially.

Raltegravir (I) thus obtained has purity above 99.5% with an over allyield ranging from 55 to 58% based on the starting material.

The present invention is a novel synthetic route for the preparation ofraltegravir which has the following advantages over prior art methods:

-   -   The present synthetic route avoids the use of protection and        deprotection steps of amine group as used in the prior art        methods;    -   Increase in the yield of pyrimidinone, methyl        2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxo        pyrimidines-4-carboxylate (Va), an important intermediate for        the synthesis of raltegravir;    -   Methylation of N        -(4-fluorobenzyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide (VI)        at nitrogen reveals regioselectivity of more than 99% over        oxygen methylation,    -   Synthetic route is cast-effective, environmentally friendly and        easily implemented on an industrial scale.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however shouldnot to be construed to be limited to the particular forms disclosed,since these are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art, withoutdeparting from the spirit of the invention.

The present invention is described herein below with reference toexamples, which are illustrative only and should not be construed tolimit the scope of the present invention in any manner.

EXAMPLES Example 1 Preparation ofN-(2-cyanopropan-2-yl)-5-methyl-1,3,4-oxadiazole-2-carboxamide (II):

2-Amino-2-methylpropanenitrile (90 gms) was suspended in dichloromethane(90 ml) and N-methyl morpholine (1.10 moles) was added with stirring. Amixture of oxadiazole carbonyl chloride (90 gms) in dichloromethane (100ml) was added drop wise to the suspension at 0 to 5° C. The reactionmixture was stirred and monitored by thin layer chromatography (TLC).The reaction mass was concentrated under reduced pressure aftercompletion of reaction. The product separated out on addition ofisopropyl alcohol (500 ml), which was then filtered and dried.

Yield: 72 gms (70%).

¹H NMR (400 MHz, CDCl₃): δ 7.34 (br s, 1H), 2.64 (s, 3H), 1.84 (s, 6H);

¹³C NMR (100 MHz, CDCl₃): δ 166.69, 158.41, 152.80, 119.90, 47.42,27.13, 11.40;

MS (CI): Calculated [for C₈H₁₀N₄O₂ (M⁺H)/z: 195.19] found: (M⁺H)/z:195.1;

Melting point: 142-145° C.

Example 2 Preparation ofN-[(1Z)-1-amino-1-(hydroxyamino)-2-methylpropan-2-yl]-5-methyl-1,3,4-oxadiazole-2-carboxamide(IVa)

N-(2-cyanopropan-2-yl)-5-methyl-1,3,4-oxadiazole-2-carboxamide (II) (185gms) was suspended in isopropyl alcohol (462 ml), heated to 50 to 55° C.Aqueous hydroxylamine (IIIa; 76 ml) was added and the reaction mixturestirred for 30 minutes at 60° C. After completion, reaction mass wascooled between 0 and 5° C. and heptane (462 ml) was added to thereaction mass. The resultant slurry was filtered, washed with heptane(185 ml) and the wet cake was dried to giveN-[(1Z)-1-amino-1-(hydroxyimino)-2-methylpropan-2-yl]-5-methyl-1,3,4-oxadiazole-2-carboxamide(IVa).

Yield: 198 gms (94%).

¹H NMR (400 MHz, DMSO d⁶): δ 9.36 (br s, 1H), 8.75 (s, 1H) 7.43 (br s,1H) 5.58 (s, 2H), 2.55 (s, 3H), 1.55 (s, 6H);

¹³C NMR (100 MHz, DMSO d⁶) δ 165.63, 158.91, 155.13, 151.76, 54.95,24.86, 10.74;

MS (CI): Calculated [for C₈H₁₃N₅O₃ (M⁺H/z: 228.22], found: (M⁺H)/z;228.1.

Melting point: 140-150° C.;

Example 3 Preparation ofmethyl-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate(Va)

N-[(1Z)-1-amino-1-(hydroxylamino)-2-methylpropan-2-yl]-5-methyl-1,3,4-oxadiazole-2-carboxamide(IVa) (198 gms) was suspended in methanol (1188 ml) and cooled to 15 to25° C.. Dimethyl acetylenedicarboxylate (DMAD; 152.8 gms) was added andthe reaction mass was stirred for 2 to 3 hours at 25° C. The reactionmass was concentrated under reduced pressure and xylene was added andstirred between 135° C. and 125° C. for 6 hour. After completion ofreaction, the mixture was cooled to 60° C. and methanol (170 ml) &methyl tert-butyl ether (MTBE) were added to the reaction mass andstirred for 1 hour. The resultant slurry was filtered and washed with a9:1 mixture of methanol & methyl tert-butyl ether (MTBE) and dried togive methyl2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate(Va).

Yield: 198 gms (66%).

¹H NMR (400 MHz, DMSO d⁶): δ 12.74 (s, 1H), 10.35 (s, 1H), 9.12 (s, 1H),3.81 (s, 3H), 2.58 (s, 3H), 1.59 (s, 6H);

¹³C NMR (100 MHz, DMSO d⁶): δ 166.60. 166.15, 160.19, 159.23, 153.26,152.87, 145.65, 128.30, 56.60, 52.91, 26.26, 11.34;

MS (CI): Calculated [for C₁₃H₁₅N₅O₆ (M⁺H)/z: 338.29], Found: (M⁺H)/z;338.2.

Melting Point: 229-234° C.;

Example 4 Preparation ofN-(4-fluorobentyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide(VI):

Methyl2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxylate[V(a); 40 gms] was suspended in isopropyl alcohol (400 ml) and4-fluorobenzyl amine (30 gms) was added to the above mixture. Thereaction mixture was stirred at 80 to 85° C. After completion, reactionmass was cooled to 60° C. and acetic acid (1600 ml) and water (400 ml)was added. The resultant crystalline product was filtered and washedwith mixture of water and isopropyl alcohol (1:1) and dried to giveN-(4-fluorobenzyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide VI),

Yield: 46 gms (90%).

³H NMR (400 MHz, DMSO d⁶ ) δ 12.54 (s, 1H), 12.34 (s, 1H), 9.20-9.18 (t,1H), 9.04 (s, 1H), 7.40-7.37 (m, 2H), 7.19-7.14 (m, 2H), 4.50-4.48 (d,2H), 2.58 (s, 3H), 1.63 (s, 6H);

¹³C NMR (100 MHz, DMSO d⁶) δ 149.95, 14526, 132.66, 131.75, 128.54,128.36, 126.11, 125.10, 119.70, 117.16, 115.94, 109.62, 47.87, 47.68,45.30, 42.90;

MS (CI): Calculated [for C₁₉C₁₉FN₆O₅ (M⁺H)/z: 431.39], found: (M⁺H)/z:431.2.

Melting point: 213-216° C.;

Example 5 Preparation of Raltegravir (I):

N-(4-fluorobenzyl)-2-(2-(5-methyl-1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-1,6-dihydro-5-hydroxy-6-oxopyrimidine-4-carboxamide (VI)(50 gms), magnesium hydroxide (13.9 gms) trimethylsulfoxonium iodide(52.4 gms) and water (1.3 ml) were heated to 100° C. for 7 hour. Thereaction was cooled to 20° C. and stirred for 15 minutes. An aqueoussolution of sodium bisulfate (2M; 5 ml) was added and stirred for 30 to45 minutes at 25 to 35° C., reaction mass was extracted withdichloromethane (400 ml) and solvent concentrated under reducedpressure. The product was isolated by crystallization from mixture ofmethanol-water (1:1). The resultant slurry was filtered and washed withmixture of methanol-water (50 ml) and dried to give Raltegravir (I).

Yield: 46 gms

Yield: 94%.

1.-19. (canceled)
 20. A compound of formula:

wherein R is CN;

wherein R¹ is H or CH₃;

wherein R² is H and R³ is C₁₋₄ alkyl group; or


21. A compound as recited in claim 20 of the formula


22. A compound as recited in claim 20 of the formula

wherein R² is H or CH³.
 23. A compound as recited in claim 20 of theformula

wherein R² is H and R³ is C₁₋₄ alkyl group.
 24. A compound as recited inclaim 20 of the formula